1. Field of the Invention
The present invention relates to a split-flow flowmeter for measuring parameters related to flow, particularly flow rate and flow velocity. More particularly, the invention relates to a split-flow flowmeter using an exothermic or endothermic detection element and/or a detection element integrally formed on a semiconductor chip; for example, a split-flow flowmeter favorably applicable to a mass flow sensor for use in combustion control of a vehicle engine or an industrial engine, a mass flow sensor for use in an industrial air-conditioning system or a compressed-air supply system, or a flow sensor for use in control of the air-fuel ratio of a household gas cooker.
2. Description of the Related Art
In recent years, the circumstances surrounding automobiles have shifted toward stronger consideration for the environment, as has been demanded by emission regulations and the like. In order to comply with such regulations, engine combustion must be controlled with higher accuracy, and in this connection a flowmeter must be able to accurately measure a flow rate within a suction pipe.
Conventionally, a split-flow flowmeter has been proposed as a flowmeter for measuring a flow rate within a suction pipe. The split-flow flowmeter detects a portion of flow diverted from a main-flow pipe (an object pipe of measurement) into a flow path thereof (hereinafter also referred to as a xe2x80x9csplit-flow passagexe2x80x9d or a xe2x80x9cflow splitter tubexe2x80x9d) to thereby measure a flow rate in the main-flow pipe.
3. Problems to be Solved by the Invention
However, a conventionally proposed split-flow flowmeter involves the following problems: flow disturbance on a detection element is great; in particular, the measurement accuracy is low in measuring a low-flow-rate region.
It is therefore an object of the present invention to provide a split-flow-type flowmeter capable of reducing flow disturbance in the vicinity of a detection element within a flow path thereof and capable of accurately measuring flow rate in a low-flow-rate region.
The above objects of the present invention have been achieved by providing a flowmeter for detecting a portion of flow diverted from a main-flow pipe, which is an object of measurement, into the same, using a detection element so as to measure parameters related to the flow within the main-flow pipe; i.e., a split-flow flowmeter, which comprises a detection element disposed to face a flow path through which a fluid to be measured flows, and a venturi structure adapted to throttle a flow directed toward the detection element to thereby reduce disturbance of the flow. The split-flow flowmeter is characterized in that the venturi structure is disposed opposite the detection element within the flow path and comprises a protrusion protruding toward the detection element within the flow path.
This split-flow flowmeter is mainly characterized in that a portion of the venturi structure assumes the form of a protrusion. However, the entire venturi structure (the venturi structure itself) may assume the form of a protrusion. In such a split-flow flowmeter, the venturi structure reduces flow disturbance in the vicinity of the detection element within the flow path of the split-flow flowmeter, whereby a flow rate can be accurately measured with small variations in output. Further, the protrusion reduces a region where resistance to flow arises, and can generate a large pressure difference, whereby the rate of flow passing through a clearance between the protrusion and the detection element can be easily increased even in measuring a low-flow-rate region, thereby greatly enhancing output. Further, in this split-flow-type flowmeter, since a region where resistance to flow arises is reduced, output is also enhanced in measuring in a medium-flow-rate region and a high-flow-rate region. Additionally, since throttling yields the effect of reducing disturbance, measurement can be carried out at high resolution, whereby a flow rate can also be accurately measured in medium- and high-flow-rate regions.
A preferred mode for carrying out the present invention will next be described.
According to a preferred mode for carrying out the present invention, a venturi structure partially protrudes (a protrusion is formed on the venturi structure); i.e., a portion of the venturi structure is formed into a protrusion. The protrusion is disposed opposite a detection element within a flow path of a split-flow-type flowmeter and protrudes toward the detection element within the flow path. An example of the detection element may be a diaphragm type in which a heater and two temperature sensors are formed for detecting a gas flow (as described in U.S. patent application Ser. No. 09/754,343, incorporated herein by reference, or European Patent Application No. 01300077.3 filed by the present applicant).
Preferably, the venturi structure is configured such that the outside wall of a bottom portion of a curved partition, which is disposed within a split-flow passage so as to impart substantially the shape of the letter xcexa9 to the passage, is disposed in the opposing proximity of the detection element. A protrusion is formed which protrudes toward the detection element from the outside wall of the curved partition. A height (B) of the protrusion is preferably in the range of 0.5-4.5 mm, a gap or clearance (A) between the protrusion and the detection element is preferably in the range of 0.3-3 mm and a thickness (Dt) of the protrusion is preferably in the range of 0.5-3 mm not exceeding the width or length of the detection element as measured in a gas flow direction, as may be understood by FIG. 9. Preferably, the protrusion has a ridge-like shape as shown in FIG. 11, in which a ridge length (H) is preferably in the rage of 1-10 mm and the length of the detection element is within the ridge length (H). Importantly, the protrusion protrudes sharply. In other words, a convex radius (C) formed at a bottom skirt of the protrusion as shown in FIG. 9 is about less than 2 mm. Each of these dimensional factors contributes to a sensitive detection of the pressure variation of a gas flowing over the detection element.
According to the preferred mode for carrying out the present invention, a portion or the entirety of a surface of the protrusion which faces the detection element is curved. This feature prevents the generation of vortexes or separation of flow in the vicinity of the protrusion, and thus disturbance of air flow is reduced, thereby enhancing output.
According to the preferred mode for carrying out the present invention, a section of the protrusion taken along a plane extending in a flow direction in a flow path (split-flow passage) of a split-flow flowmeter assumes, singly or in combination, any of a triangular shape, a rectangular shape, a polygonal shape, a spindle shape, a semicircular shape, and a semielliptic shape. This feature can greatly reduce a region where resistance to flow arises, thereby enhancing output in measurement not only in a low-flow-rate region but also in a high-flow-rate region.
According to the preferred mode for carrying out the present invention, the venturi structure comprises a plurality of protrusions. Employing a plurality of protrusions in combination can impart the desired flow rate characteristics to the flowmeter.
According to the preferred mode for carrying out the present invention, the flow path structure of the split-flow flowmeter is formed to have symmetry with respect to a plane including the centerline perpendicular to a detection portion of the detection element. Particularly, the plurality of protrusions are arranged symmetrically with respect to the plane. Such a split-flow-type flowmeter can equivalently measure a regular flow and a backflow.
According to the preferred mode for carrying out the present invention, the plurality of protrusions are formed or arranged opposite one another along the flow transversal direction of the flow path of the split-flow flowmeter. A groove or a space is formed between the protrusions and functions to guide a flow toward the detection portion of the detection element.
Preferably, a groove or a space which opens toward the detection portion of the detection element is formed between the plurality of protrusions. This feature allows sufficient flow rate in measuring a low-flow-rate region, and thus output is enhanced in measuring a low-flow-rate region. Since a throttle is formed in a very small region, a flow can only be led to the detection portion of the detection element while a region where resistance to flow arises is reduced. Additionally, since a large pressure difference can be generated, flow rate can be increased in measuring a low-flow-rate region, thereby enhancing output.
Preferably, a high protrusion which protrudes toward the detection element is formed between the plurality of protrusions. In measuring a low-flow-rate region, throttling effected by the central high protrusion enhances output. In measuring a high-flow-rate region, low protrusions located on either side of the central high protrusion provide bypasses in order to delay saturation to thereby increase output during measurement of high flow rate.
According to the preferred mode for carrying out the present invention, the clearance between the tip of the protrusion and the detection element is equal to or smaller than the length of the protrusion as measured along a direction parallel with the surface of the diaphragm of the detection element and perpendicular to a flow direction (a length as measured along the flow transversal direction). For example, when the protrusion length is not greater than 3 mm as measured along the flow transversal direction, the clearance between a tiptop of the protrusion and the detection element is preferably not greater than 3 mm. The above-described relationship between the surface of the diaphragm of the detection portion and an opposite surface length (of the protrusion) effects sufficient throttling at an opening portion of the venturi, thereby yielding the desired effects.
A split-flow-type flowmeter according to the present invention can measure not only flow rate but also parameters related to flow, such as flow velocity, as needed.
In order to realize stable measurement at high accuracy, the preferred mode for carrying out the present invention comprises a bypass flow path straightly connecting the flow inlet and the flow outlet and bypassing the split-flow passage and/or a venturi structure for decreasing the diameter of the split-flow passage in the vicinity of the detection element. The bypass flow path stabilizes supply of fluid to be measured to the detection element and facilitates diversion of fluid to be measured (flow in the main-flow pipe) into the split-flow passage. The venturi structure effectively rectifies turbulence of fluid to be measured which would otherwise arise on a detection portion (also referred to as a detection surface) of the detection element. Thus, even when pulsation or pulsation plus backflow is generated, the bypass flow path and the venturi structure stabilize measurement and enable measurement with high accuracy.
Particularly, in the case where the split-flow passage assumes a symmetrical structure such that the inlet side and the outlet side are symmetrical with respect to the detection element, employing a venturi structure for decreasing the size of the bypass flow path or the diameter of a flow cross section of the bypass flow path further stabilizes flow reaching the detection element even when pulsation or pulsation plus backflow is generated.
According to the preferred mode for carrying out the present invention, a venturi structure is disposed in the bypass flow path to thereby determine the flow rate of fluid to be measured and diverted toward the detection element, by means of the amount of projection of a flow path wall which constitutes the venturi structure or the area of opening of the venturi structure. Thus, the flow rate of flow heading for the detection element can be quantitatively controlled.
According to the preferred mode for carrying out the present invention, flow control means for forming flow hitting obliquely on the detection surface of the detection element is provided in the split-flow passage. The flow control means causes steady flow onto the detection surface of the detection element, so that flow to be detected reliably flows on the detection surface. Additionally, since generation of vortexes and separation of flow in the vicinity of the detection surface is suppressed, detection accuracy and reproducibility are enhanced.
According to the preferred mode for carrying out the present invention, flow control means for forming flow hitting obliquely on the detection surface of the detection element or forming flow flowing obliquely with respect to the detection surface assumes the form of a flow path surface (an elevated portion) elevated above the detection surface, which elevated flow path is located at least upstream of the detection element, or upstream and/or downstream of the detection element. The form of elevation is not particularly limited so long as a flow hitting obliquely on the detection surface is formed. Preferably, the form of elevation is concave or convex, or the elevated surface is a linear, polygonal, or concavely-curved slant surface.
According to the preferred mode for carrying out the present invention, the detection surface of the detection element is exposed to the interior of the split-flow passage (detection tube) at an inflection portion of the split-flow passage. Preferably, an inflection tube (split-flow passage) is attached to the main-flow pipe (a pipe at which measurement is performed) in a perpendicularly intersecting condition, and the detection element is disposed at an inflection portion (a bent portion, or a curved portion of flow path) of the inflection tube. Alternatively, the detection element is disposed at or in the vicinity of a portion of the split-flow passage where flow is inverted or the direction of flow is greatly changed. Preferably, the detection surface of the detection element is exposed to a portion of the interior of the split-flow passage where flow is fast. Preferably, the detection surface of the detection element is exposed to a portion, or its vicinity, of the interior of the split-flow passage where flow is throttled and then changes its direction.
According to the preferred mode for carrying out the present invention, the detection element mounted on the bottom wall of the split-flow passage (a wall of the flow path located farthest away from the flow inlet and the flow outlet) is located outside the main-flow pipe. Thus, the detection element can be readily mounted or replaced. Also, output from the detection element is readily released.
The preferred mode for carrying out the present invention can use the following detection element. Specifically, the detection element is a thermal detection element comprising a substrate and four thin-film resistors formed on the substrate. More specifically, a diaphragm section and a rim section are formed on a semiconductor substrate. The diaphragm section includes (1) an upstream temperature sensor, (2) a downstream temperature sensor, and (3) a heater disposed between the upstream temperature sensor and the downstream temperature sensor. The rim section includes (4) an ambient temperature sensor. The diaphragm section is finished very thinly and is thermally insulated.
Next will be described the principle of detection of flow-related parameters, such as flow velocity and flow rate, by use of the detection element.
(1) Power supplied to the heater is controlled such that a constant difference is maintained between the temperature of the heater and the ambient temperature.
(2) Thus, when flow is not present, the upstream temperature sensor and the downstream temperature sensor indicate substantially the same temperature.
(3) However, when flow is present, heat escapes from the surface of the upstream temperature sensor; thus, the temperature of the upstream temperature sensor is reduced. Because of an increase in thermal input from the heater, a temperature change of the downstream temperature sensor is smaller than that of the upstream temperature sensor. Notably, in some cases, the temperature of the downstream temperature sensor may rise.
(4) Flow rate, flow velocity, or a like parameter is detected on the basis of the temperature difference between the upstream temperature sensor and the downstream temperature sensor. The direction of flow is detected from the sign of the temperature difference (magnitude relation). Notably, the temperature difference can be detected on the basis of a change in electrical resistance caused by temperature.
The preferred mode for carrying out the present invention can use another detection element as follows. Specifically, the detection element is a thermal detection element comprising a substrate and three thin-film resistors formed on the surface of the substrate. More specifically, a diaphragm section and a rim section are formed on a semiconductor substrate. The diaphragm section includes (1) an upstream heater and (2) a downstream heater. The rim section includes (3) an ambient temperature sensor. The diaphragm section is finished very thin and thermally insulated.
Next the principle of detection of flow-related parameters will be described, such as flow velocity and flow rate, by use of the detection element.
(1) Power supplied to the upstream and downstream heaters is controlled such that a constant difference is maintained between the upstream/downstream heater and the ambient temperature.
(2) Thus, when flow is not present, the upstream heater and the downstream heater indicate substantially the same temperature.
(3) However, when flow is present, heat escapes from the surfaces of the upstream and down stream heaters; thus, the temperature of the upstream and downstream heaters drops. Because of an increase in thermal input from the upstream heater, a temperature change of the downstream heater is smaller than that of the upstream heater. Notably, in some cases, the temperature of the downstream heater may rise.
(4) Flow rate, flow velocity, or a like parameter is detected from the difference in current or voltage required to maintain a constant temperature between the upstream heater and the downstream heater as obtained on the basis of a temperature drop of each of the upstream and downstream heaters. The direction of flow is detected from the sign of the current or voltage difference (magnitude relation). Notably, the temperature drop can be detected on the basis of a change in electrical resistance caused by temperature.
According to the preferred mode for carrying out the present invention, the detection element measures flow-related values, such as flow rate and/or flow velocity, on the basis of temperature.
The split-flow-type flowmeter according to the present invention can be installed in an intake system of an engine to be mounted in various kinds of vehicles, two-wheeled and four-wheeled, in order to measure an intake rate or a like parameter. For example, the split-flow-type flowmeter according to the present invention is installed in an intake system of an engine to be mounted in a four-wheeled vehicle, somewhere on a pipe line extending between the interior of an air cleaner and a throttle valve. The split-flow-type flowmeter according to the present invention is installed in an intake system of an engine to be mounted on a two-wheeled vehicle; specifically, on an intake pipe connected to a cylinder, on an air funnel within an air cleaner box, or on a like location, in order to measure an intake gas flow rate, an intake gas flow velocity, or a like parameter.